The invention relates to all kinds of heat treatment of objects in which an object is received in a susceptor and heated by heating the susceptor. Devices presently known for such heat treatments involve a number of problems, which may, by way of a non-limitative example, be illustrated in the case of epitaxially growing crystals, i.e. single crystals, by Chemical Vapor Deposition (CVD). Furthermore, the problems commonly encountered in growing crystals by CVD will be discussed below for the particular case of growing crystals of SiC, but similar results can be obtain for growing crystals of other materials, such as, in particular, Group III-nitrides and all types of alloys of such nitrides and/or SiC. This growth is mostly carried out while, at the same time, intentionally doping the object grown.
SiC single crystals are, in particular, grown for use in different types of semiconductor devices, such as for example different types of diodes, transistors and thyristors, which are intended for applications in which it is possible to benefit from the superior properties of SiC in comparison with especially Si, namely the capability of SiC to function well under extreme conditions. The large bandgap between the valance band and the conduction band of SiC makes devices fabricated from Sic material operable at high temperatures, namely up to 1000.degree. K.
However, high temperatures are needed for obtaining a good, orderly growth. The epitaxial growth of silicon carbide by Chemical Vapor Deposition is therefore carried out in a temperature regime in excess of 1400.degree. C. These high temperatures are needed both to obtain decomposition by cracking of Si- and C-containing precursor gases of the gas mixture used for the CVD, and to ensure that the atoms are deposited on the surface of the substrate on which the crystal is grown in an ordered manner. However, high temperatures also present problems, such as having impurities coming out of different types of material, so that the temperature could not, until now, for the growth of SiC be raised above a temperature interval of 1400.degree.-1700.degree. C., necessarily resulting in a rather low growth rate (some .mu.m per hour) so that it is impratical to grow boules for forming i.e. substrates, by using CVD. Consequently, this method is only used for growing objects in the form of layers. However, it is even possible to grow layers of SiC by CVD through devices already known at such a high growth rate that a commercial production thereof becomes commercially interesting.
A raise of the temperature for increasing the growth rate could, until the present, not be used, since this would result in rapid degradation of the walls of the susceptor due to increased etching of hot spots thereof, leading to an unacceptable incorporation of impurities therefrom into the grown layers.
As already mentioned, it is, due to the low growth rates, impossible to grow boules, which require growth rates in the order of millimeters per hour, by CVD. Consequently the seeded sublimation technique is presently used for growing boules, which may then be sliced to substrates. However, the crystalline quality of the boules grown by this technique is low in comparison with that of the SiC layers epitaxially grown by CVD. The substrates so produced are perforated with small holes called micropipes or pinholes, which limit the device area considerably which consequently make high-power devices of SiC not commercially practical. In the seeded sublimation technique the source is a SiC powder that sublimes, whereupon the gas species are transported by a temperature gradient to the seed crystal where the growth occurs. The conditions in the gas phase are governed by thermodynamics only, which makes it difficult to keep the C/Si ratio constant due to Si vapor leakage out of the system. Furthermore, the purity of the gases used for Chemical Vapor Deposition are several orders of magnitude higher than that of the source material used for seeded sublimation growth.
The prior art includes devices using susceptors of graphite, which contain major amounts of unwanted impurities with a tendency to be released and deposited on the crystal grown when the susceptor is heated. This problem has been partially solved by applying a SiC-coating on the susceptors and, additionally, a plate of SiC can be placed underneath the substrate. However, after a while the SiC-coating of the susceptor may be removed, especially by etching, and impurities are introduced into the gas-phase. Cracks in the SiC-coating may also be produced due to stresses arising between the different materials, namely SiC and the graphite, as a result of temperature changes.
In most cases when other crystals than SiC are grown the temperature is lower and therefore the impurity problems due to high temperatures are not as accentuated as for SiC, but the problems with etching of the susceptor or protective coating thereof, resulting in cracks of such a coating thus remain.
As already mentioned, the invention is applicable to all types of heat treatments of this kind. The problem that the susceptor sets an upper limit for the temperature that may be used for the heat treatment as well as the problem with impurities coming out from the susceptor are in common therefore.